1. Technical Field
The present invention pertains to communication systems. In particular, the present invention pertains to a channel reservation communication system employing reliable transmission of broadcast messages (e.g., messages transmitted from a system source node to plural destination nodes within the system).
2. Discussion of Related Art
Generally, wireless communication systems or networks employ radio channels that are shared by each radio node within a system. However, simultaneous transmission of messages by radio nodes over the same radio channel produces collisions that degrade system performance. These collisions basically prevent reception of messages by the nodes, thereby causing the messages to be dropped and/or re-transmitted. Various conventional channel access schemes or protocols may be employed by a communication system to reduce the occurrence of collisions, such as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), Time Division Multiple Access (TDMA) and channel reservation.
With respect to channel reservation, these types of schemes are typically employed by wireless communication systems to enhance reliability of packet transmissions. Basically, channel reservation schemes utilize one or more channels for transferring data and an additional independent channel for transference of reservation control information (e.g., information to reserve a data channel for transference of the data), and typically conduct a two-way handshake to initiate the data transference. In particular, a source node communications device, such as a modem, initially reserves a data channel for transmission of a data packet in response to the data packet being received from a succeeding layer of a communications protocol employed by the system. Communications protocols typically include hierarchical layers with each layer having a specific function that, in combination, receive, process and transmit messages. The layer functions may range from packet generation and processing to physical or hardware functions of facilitating communications across the network. Channel reservation is accomplished by the source node transmitting a Request-to-Send (RTS) packet to an intended destination node over the reservation channel in response to detecting available data channel space. The RTS packet generally includes information that informs the destination node of a pending data packet and permits the destination node to determine the presence of sufficient available data channel space for the source node to transmit that packet. If the destination node detects sufficient available channel space for transmission of the pending data packet, a Clear-to-Send (CTS) packet is transmitted over the reservation channel from the destination node to the source node and includes appropriate information to enable reservation of the data channel space for transmission of the data packet. Once the source node receives the CTS packet, the data packet is transmitted from the source node to the destination node over the reserved data channel space. Since the destination node determines appropriate conditions for transmission of data packets, the probability of collisions due to xe2x80x9chidden terminalsxe2x80x9d (e.g., nodes that simultaneously transmit messages to a common destination node without being aware of the other transmissions, thereby causing collisions at the common destination node) is minimized.
The channel reservation technique described above typically enhances communications for point-to-point messages (e.g., messages that are directed from a source node to one specific destination node). However, with respect to transmission of broadcast messages (e.g., messages transferred from one source node to plural destination nodes), this technique suffers from several disadvantages. In particular, the identities of intended destination nodes for a broadcast message are typically unknown to a source node (e.g., at the physical layer of the communications protocol where messages are transferred onto the network), thereby imposing a substantial barrier for addressing and transmitting RTS packets to different destination nodes. Further, even if destination nodes for a broadcast message are identified by a source node, a substantial quantity of CTS messages are sent by the destination nodes over the reservation channel in response to receiving an RTS packet, thereby causing collisions in the reservation channel and reducing the success rate of reserving suitable data channel space.
Since broadcast messages cannot be efficiently transmitted utilizing conventional channel reservation schemes as described above, different techniques may be more suitable for handling those types of messages. For example, one approach may include transmitting the broadcast message via a modified channel reservation scheme. In particular, a source node initially transmits an RTS message over a reservation channel to all nodes within the source node transmission range. Once the RTS message is received, the destination nodes wait for transmission of the broadcast message over a data channel identified in the RTS packet, while the source node waits for expiration of a predetermined time interval. When the time interval expires, the source node transmits the broadcast message over the identified data channel.
The above-described techniques suffer from several disadvantages. In particular, the techniques described above generally do not provide reliable broadcast communications and adversely affect channel conditions. Since channel space for transmission of broadcast messages cannot be securely reserved, transmission of these messages causes collisions in the data channel when there is transmission of other data packets. Although collisions within the data channel are infrequent when employing the channel reservation techniques described above with a low or light traffic load, the frequency of collisions with these techniques increases with heavier traffic loads, thereby causing re-transmission of point-to-point messages and congestion of the data channels. Thus, performance of the above-described techniques depends upon the condition of the reservation and data channels. In other words, these techniques enhance communications when channel use is low, but have poor performance and degrade channel conditions when the channels are heavily utilized or congested.
Accordingly, it is an object of the present invention to broadcast messages within a communications system while minimizing performance degradation due to increased system traffic.
It is another object of the present invention to enhance reliability of broadcast transmissions within a communication system and maintain system performance at a desired level.
Yet another object of the present invention is to utilize redundant Request-to-Send type messages (BRTS) in a communication system to ensure reliability of broadcast transmissions.
Still another object of the present invention is to dynamically adjust the quantity of RTS type messages (BRTS) transmitted within a communication system to enhance reliability of broadcast transmissions and control system performance.
The aforesaid objects may be achieved individually and/or in combination, and it is not intended that the present invention be construed as requiring two or more of the objects to be combined unless expressly required by the claims attached hereto.
According to the present invention, a broadcast message is transmitted within a wireless communication system or network via utilization of redundant Request-to-Send type or broadcast notice messages or packets (BRTS). A source node repeatedly transmits a sequence of these messages over a system reservation channel with each message containing a sequence identifier. The sequence identifier of each succeeding message has a value immediately preceding the identifier of the previously transmitted broadcast notice message. A destination node receives the messages and determines the transmission time of the broadcast message based on each received message identifier. Subsequent transmission and reception of the final broadcast notice message in the sequence, the broadcast message is transmitted by the source node to the destination node over a data channel identified in the transmitted messages. The quantity of broadcast notice messages may be dynamically adjusted to maintain system performance at a desired level.
The present invention achieves a greater success rate for transmission of broadcast messages than conventional techniques that provide limited channel reservation capability, and has a system performance that degrades gracefully and to a lesser degree than those techniques as the channels become congested. Further, the present invention achieves greater transmission reliability, and by dynamically controlling parameters or the quantity of the broadcast notice messages employed, provides a manner in which to maintain a pre-defined success rate under dynamically changing conditions. In other words, transmission performance of the present invention may be maintained as channels become congested by dynamically adjusting parameters. The present invention basically utilizes redundant broadcast notice messages to ensure broadcast transmission reliability, thereby enhancing transmission performance and reducing reservation channel capacity. Since utilization of the reservation channel is generally low, increase in reservation channel traffic due to the redundant broadcast notice messages has a limited impact on the reservation channel.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of specific embodiments thereof, particularly when taken in conjunction with the accompanying drawings, wherein like reference numerals in the various figures are utilized to designate like components.